1. Technical Field
In general, the present invention relates to a silicon carbide (SiC) lateral metal-oxide-semiconductor field-effect transistor (LMOSFET). In particular, the present invention provides a SiC LMOSFET having a self-aligned drift region and method for forming the same.
2. Background Art
In recent years, silicon (Si) lateral diffused metal-oxide semiconductor field effect transistors (LDMOSFETs) have gained wide acceptance in a variety of high power and high frequency applications. Si LDMOSFETs offer simpler gate drive and lower signal distortion in comparison to bipolar-based devices. LDMOSFETs in the silicon field are made using self-aligned technology to reduce the overlap between the gate and source/drift regions. The reduction of this overlap is important as it reduces the gate-source and gate-drain capacitance, which can adversely affect the high frequency performance of the device. It is also desirable to reduce the overlap to decrease the cell pitch and conserve the silicon area used by the device.
In producing an LDMOSFET using self-aligned technology, a gate contact is formed on top of a gate oxide layer. Once formed, an N+ source region and a P base region are diffused from the source side of the device while an Nxe2x88x92 drift region is diffused from the drain side. This diffusion makes the source, base, and drift regions self aligned to the gate electrode. Since the diffusion temperature in Si technology is about 900-1100xc2x0 C., neither the gate electrode nor the gate oxide layer is affected in any adverse fashion.
More recent advancements in technology have lead to the use of SiC as a substitute for Si in constructing lateral MOSFETs for high frequency and high power applications. However, when using SiC, diffusion of dopants is extremely difficult. Hence, only high-energy ion implantation can be used, which requires annealing temperatures in excess of 1500xc2x0 C. Such high temperatures are extremely harmful to the gate oxide layer and the gate electrode. Accordingly, the source, drain, and drift regions must be implanted and annealed prior to formation of the gate oxide layer and gate electrode. This technology is referred to as non self-aligned technology and results in larger overlaps for both the source and drift regions with the gate electrode. As indicated above, as overlap increases, the capacitance of the device also increases. Higher capacitance adversely affects device performance. This is especially the case with respect to the overlap between the drift region and the gate contact since the drain to gate capacitance is a feedback capacitance (i.e., Miller capacitance).
In view of the foregoing, there exists a need for a SiC LMOSFET in which the source, drain, and drift regions can be implanted without adversely affecting the gate oxide layer or gate electrode. In addition, a need exists for the drift region to be self-aligned with the gate electrode so that the overlap between the gate contact and drift region is minimized. By minimizing the overlap, the capacitance of the device is minimized and performance is improved.
The present invention solves the problems with existing art by providing a silicon carbide (SiC) lateral metal-oxide semiconductor field-effect transistor (LMOSFET) in which the drift region is self-aligned with the gate electrode. By self-aligning the drift region with the gate electrode, the drain to gate capacitance is substantially reduced, which improves performance of the device. In general, the SiC LMOSFET is formed by implanting the N+ source and drain regions in an epitaxial layer. After implantation, the regions are annealed at high temperatures. Then, the gate oxide layer and gate electrode are formed. Once the gate electrode has been formed, the Nxe2x88x92 drift region is implanted and then annealed. Since the drift region is doped with a lower implant dose, lower annealing temperatures can be used. Such lower temperatures do not seriously affect the gate oxide layer or gate electrode. Moreover, since the drift region is implanted after formation of the gate electrode, the drift region is self-aligned with the gate electrode (i.e., the overlap is minimized).
According to a first aspect of the present invention, a silicon carbide lateral metal-oxide-semiconductor field-effect transistor (SiC LMOSFET) is provided. The SiC LMOSFET comprises: (1) a source region, a drain region, and a drift region implanted in an epitaxial layer; and (2) a gate electrode formed over the epitaxial layer, wherein the drift region is self-aligned with the gate electrode.
According to a second aspect of the present invention, a silicon carbide lateral metal-oxide-semiconductor field-effect transistor (SiC LMOSFET) is provided. The SiC LMOSFET comprises: (1) a silicon carbide substrate; (2) an epitaxial layer formed on the silicon carbide substrate; (3) a source region, a drain region, and a drift region implanted in the epitaxial layer; (4) a gate electrode formed over the epitaxial layer; and (5) wherein the drift region is self-aligned with the gate electrode, and wherein the source region is non self-aligned with the gate electrode.
According to a third aspect of the present invention a method for forming a silicon carbide lateral metal-oxide-semiconductor field-effect transistor (SiC LMOSFET) is provided. The method comprises the steps of: (1) implanting a source region and a drain region in an epitaxial layer; (2) annealing the implanted source region and the implanted drain region; (3) forming a gate oxide layer and a gate electrode over the epitaxial layer; (4) implanting a drift region in the epitaxial layer; and (5) annealing the implanted drift region.
According to a fourth aspect of the present invention a method for forming a silicon carbide lateral metal-oxide-semiconductor field-effect transistor (SiC LMOSFET) is provided. The method comprises the steps of: (1) implanting a source region and a drain region in an epitaxial layer, wherein the epitaxial layer is formed on a silicon carbide substrate; (2) annealing the implanted source region and the implanted drain region at approximately 1600xc2x0 C.; (3) forming a gate oxide layer over the epitaxial layer after the source region and the drain region are annealed; (4) forming a gate electrode on the gate oxide layer; (5) implanting a drift region in the epitaxial layer after the gate electrode is formed; and (6) annealing the implanted drift region at approximately 1250xc2x0 C.
Therefore, the present invention provides a SiC LMOSFET having a self-aligned drift region and a method for forming the same.